1. Field of the Invention
The present invention relates to a semiconductor device having an insulated gate type field effect transistor and a method for producing the same, and more particularly, to an insulated gate type field effect transistor forming one cell of a semiconductor memory device and a method for producing the same.
2. Description of the Prior Art
In recent years, the degree of integration of semiconductor integrated circuit devices and especially of semiconductor memory devices has become higher and higher, and accordingly miniaturization of an insulated gate type field effect transistor to be used in such devices becomes essentially necessary. The respective field effect transistors in an integrated circuit device are formed within active regions substantially surrounded by an isolating region including a thick field oxide film. Each transistor includes source and drain regions and a channel region between these two regions, or one of the source and drain regions and channel region extending from that one region. In the prior art device, the edges of the channel region in the widthwise direction, that is, in the perpendicular direction with respect to a direction from the source to the drain, are contiguous to an isolating region which has the same construction as the isolating region adjacent to the source and drain regions, and therefore the widthwise dimension of the channel region is determined by the surrounding isolating region. As the transistors become further miniaturized, the widthwise dimension of the channel region becomes as small as several microns, and it becomes impossible to precisely realize a predetermined dimension of the channel region in the widthwise direction by means of the isolating region including a thick field oxide film of about 1 .mu.m in thickness. In addition, where the isolating region includes a so-called channel stopper region which is formed at the surface of the semiconductor substrate directly under the thick field oxide film and has a higher impurity concentration than and the same conductivity type as the substrate, this high concentration channel stopper region inevitably protrudes into the channel region when the thick field oxide film is formed by thermal oxidation. Since the extent of this protrusion depends upon the conditions of the above-described thermal oxidation and the thickness of the oxide film, it is impossible to control the widthwise dimension of the channel region precisely at a desired dimension. Furthermore, the width edges of the channel region are defined by the insulative isolating region including the thick field oxide film as described above, and consequently not self-aligned with the width edges of a gate electrode. Accordingly, the widthwise dimension of the gate electrode must be larger than that of the channel region by several microns, taking into consideration an allowance for alignment of the gate electrode with the channel region. This is disadvantageous in view of the degree of integration.